Holographic recording medium, holographic writing system and holographic reading system

ABSTRACT

A holographic recording medium including a recording layer on a substrate, which records data information in a light interference pattern. In the holographic recording medium, information on a thermal expansion characteristic of a recording material contained in the recording layer and/or information on temperature dependency of the refractive index of the recording material are recorded within the holographic recording medium in advance.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/608,614, filed on Dec. 8, 2006, which claims priority toJapanese Patent Application JP2005-357485 filed in the Japanese PatentOffice on Dec. 12, 2005 the entire contents of which being incorporatedherein by reference.

BACKGROUND

The present application relates to a holographic recording medium and aholographic writing system and holographic reading system that use theholographic recording medium.

A holographic recording scheme records by causing the interferencebetween the signal light including a two-dimensional data pattern andreference light and changing the physical properties of a recordinglayer containing a recording material in accordance with thedistribution of strength of the interference fringes.

The photopolymer functioning as the recording material is characterizedin that (1) the refractive index can be highly modulated, which canprovide a high diffraction efficiency; (2) easy processing is allowed;(3) low noise; and (4) low costs. Thus, the photopolymer is an extremelypotential material for putting the holographic recording to use.

However, photopolymer shrinks and swells largely due to a temperaturechange, which has an important effect on data reading when the datareading is performed at a different temperature from that of recordingsince the thermal shrinking/swelling of the recording layer changes thespace between the interference fringes recorded within the recordinglayer. The refractive index of a recording material also depends on thetemperature, and the change in refractive index changes the diffractioncondition, which has an important effect on data reading as well asshrinking/swelling.

The degree of the effects may depend on the difference in temperaturebetween recording and reading, the linear expansion coefficient of arecording material, the temperature characteristic of the refractiveindex, the numerical aperture of a lens, the size of a recording datapixel and the thickness of a recording layer.

By the way, in holographic data recording, the amount of photopolymercontributing to recording may be increased in order to increase therecording density, and the increase can be achieved by increasing thethickness of the recording layer. However, the increase in thickness ofthe recording layer may increase the severity of the condition (orselectivity) for the diffraction, and the effect on data reading due toa thermal deformation is larger in a thicker recording layer even with asame linear expansion coefficient.

Alternatively, the recording density can be increased by decreasing thesize of recording data pixels for increasing the data capacity of onepage or increasing the numerical aperture of a lens for decreasing thesize of the unit hologram area within a recording material. However,like the case with the recording layer with an increased thickness, thediffraction condition becomes severer due to the shrinking or swellingof a recording material.

A method has been proposed which relates to the swelling/shrinking of arecording layer due to the surrounding temperature and compensates theeffect by adjusting the laser wavelength in reading (see JP-A-2002-32001(Patent Document 1), for example).

However, the adjustment of the laser wavelength in reading is difficultwhen the characteristics of a recording material such as the linearexpansion coefficient and the temperature characteristic of therefractive index and the environment (including the temperature andwavelength) in writing are not available.

SUMMARY

Accordingly, it is desirable to propose a holographic recording mediumand a holographic writing system and holographic reading system that usethe holographic recording medium, which can properly correct the shiftof the diffraction condition due to the shrinking/swelling of arecording medium and a change in refractive index caused by a change intemperature between those in writing and reading data information andcan read data information in a stable manner.

According to an embodiment, there is provided a holographic recordingmedium including a recording layer on a substrate, which records datainformation in a light interference pattern, wherein information on athermal expansion characteristic of a recording material contained inthe recording layer and/or information on temperature dependency of therefractive index of the recording material are recorded within theholographic recording medium in advance.

In this case, the information on a thermal expansion characteristic ofthe recording material may be a linear expansion coefficient in anactually-used environment temperature area of the holographic recordingmedium.

The information on the temperature dependency of the refractive index ofthe recording material may be a primary coefficient of the ratio of achange in refractive index in accordance with a change in temperature inan actually-used environment temperature area of the holographicrecording medium.

The information on the thermal expansion characteristic of the recordingmaterial and/or the information on the temperature dependency of therefractive index of the recording material may be recorded as embosspits of the substrate.

Data information recorded on the recording layer and information on thetemperature and the writing wavelength in writing the data informationmay be recorded.

In this case, information on the temperature and writing wavelength inwriting the data information may be recorded by a holographic recordingscheme.

In this case, the information on the temperature and writing wavelengthin writing the data information may be written in larger pixel unitsthan a minimum pixel unit used for writing data information.

Information on a reference temperature and information on a referencewavelength for adjusting the wavelength of writing light for writingdata information to the recording layer or the wavelength of readinglight for reading data information from the recording layer may berecorded within the holographic recording medium in advance.

In this case, the information on the reference temperature and theinformation on the reference wavelength may be recorded as the embosspits of the substrate.

According to another embodiment, there is provided a holographic writingsystem that uses a holographic recording medium according to theembodiment above and writes data information on the holographicrecording medium by irradiating writing light to the holographicrecording medium, the system including an optical head that irradiatesthe writing light to the holographic recording medium, and temperaturemeasuring means for measuring the surrounding temperature of theholographic recording medium, wherein, in writing data information to arecording layer of the holographic recording medium, information on thesurrounding temperature measured by the temperature measuring means andthe wavelength of writing light irradiated from the optical head isrecorded within the holographic recording medium.

In this case, the information on the surrounding temperature measured bythe temperature measuring means and the wavelength of writing lightirradiated from the optical head may be recorded on the recording layerby a holographic recording scheme.

The information on the surrounding temperature measured by thetemperature measuring means and wavelength of the writing lightirradiated from the optical head may be written in larger pixel unitsthan a minimum pixel unit used for writing data information.

According to another embodiment, there is provided a holographic readingsystem that uses the holographic recording medium according to theembodiment above, the system including an optical head having readinglight creating means for creating reading light to be irradiated to theholographic recording medium by using laser light emitted from a readinglaser, wavelength control means for changing the wavelength of thereading light, and a servo laser that emits servo light having adifferent wavelength from that of the reading light, an optical systemthat irradiates the reading light or servo light to the holographicrecording medium and gathers reading light generated by the holographicrecording medium by the reading light or servo light returned by theservo light by including information from the holographic recordingmedium, an imaging element that detects the reading light, a lightdetector that detects the servo light, and temperature measuring meansfor measuring the surrounding temperature of the holographic recordingmedium, wherein data information is read from the holographic recordingmedium by irradiating the reading light to the holographic recordingmedium, and data information is read by changing the wavelength of thereading light by the wavelength control means so as to satisfy adiffraction condition on the recording layer based on information on thethermal expansion characteristic of the recording material and/orinformation on the temperature dependency of the refractive index of therecording material, which is read from the holographic recording medium,information on the temperature in writing data information andinformation on the wavelength of writing light, and information on thesurrounding temperature in reading data information, which is measuredby the temperature measuring means, and irradiating the reading light tothe holographic recording medium.

In this case, the information on the thermal expansion characteristic ofthe recording material and/or the information on the temperaturedependency of the refractive index of the recording material may be readby irradiating the servo light with the wavelength which is not sensibleby the recording material.

The information on the temperature in writing data information andinformation on the wavelength of writing light may be read byirradiating reading light by changing the wavelength so as to extractthe information.

Data information may be read by changing the wavelength λr of thereading light to the range of the values calculated by one of:0.99*((1+(α−β)*(Tr−Tw))*λw)≦λr≦1.01*((1+(α−β)*(Tr−Tw))*λw)  [1];0.99*((1+α(Tr−Tw))*λw)≦λr≦1.01*((1+α(Tr−Tw))*λw)  [2]; and0.99*((1−β(Tr−Tw))*λw)≦λr≦1.01*((1−β(Tr−Tw))*λw)  [3]

where information on the thermal expansion characteristic of therecording material is a linear expansion coefficient α, information onthe temperature dependency of the refractive index of the recordingmaterial is a primary coefficient β of the ratio of a change inrefractive index in accordance with a change in temperature, informationon the temperature in writing data information is a temperature Tw,information on the wavelength of the writing light is a wavelength λw,and information on the surrounding temperature in reading datainformation is a temperature Tr, and irradiating the reading light tothe holographic recording medium.

Data information may be read by adjusting the optical zoom in accordancewith a change in wavelength of reading light by the wavelength controlmeans.

According to another embodiment, there is provided a holographic writingsystem that uses the holographic recording medium according to theembodiment above, the system including an optical head having writinglight creating means for creating writing light to be irradiated to theholographic recording medium by using laser light emitted from a writinglaser, wavelength control means for changing the wavelength of thewriting light, and a servo laser that emits servo light having adifferent wavelength from that of the writing light, an optical systemthat irradiates the writing light or servo light to the holographicrecording medium and gathers servo light returned by the servo light byincluding information from the holographic recording medium, a lightdetector that detects the servo light, and temperature measuring meansfor measuring the surrounding temperature of the holographic recordingmedium, wherein data information is written in the holographic recordingmedium by irradiating the writing light to the holographic recordingmedium, and data information is written by changing the wavelength ofthe writing light by the wavelength control means based on informationon the thermal expansion characteristic of the recording material and/orinformation on the temperature dependency of the refractive index of therecording material, which is read from the holographic recording medium,information on a reference temperature and information on a referencewavelength, and information on the surrounding temperature in writingdata information, which is measured by the temperature measuring means,and irradiating the writing light to the holographic recording medium.

The information on the thermal expansion characteristic of the recordingmaterial and/or the information on the temperature dependency of therefractive index of the recording material and the information on thereference temperature and the information on the reference wavelengthmay be read by irradiating the servo light with the wavelength which isnot sensible by the recording material.

Data information may be written by changing the wavelength λw of thewriting light to the range of the values calculated by one of:0.99*((1+(α−β)*(Tw−Tc))*λc)≦λw≦1.01*((1+(α−β)*(Tw−Tc))*λc)  [4];0.99*((1+α(Tw−Tc))*λc)≦λw≦1.01*((1+α(Tw−Tc))*λc)  [5]; and0.99*((1−β(Tw−Tc))*λc)≦λw≦1.01*((1−β(Tw−Tc))*λc)  [6]

where information on the thermal expansion characteristic of therecording material is a linear expansion coefficient α, information onthe temperature dependency of the refractive index of the recordingmaterial is a primary coefficient β of the ratio of a change inrefractive index in accordance with a change in temperature, informationon the reference temperature is a temperature Tc, information on thereference wavelength is a wavelength λc, and information on thesurrounding temperature in writing data information is a temperature Tw,and irradiating the writing light to the holographic recording medium.

According to another embodiment, there is provided a holographic readingsystem that uses the holographic recording medium according to theembodiment above, the system including an optical head having readinglight creating means for creating reading light to be irradiated to theholographic recording medium by using laser light emitted from a readinglaser, wavelength control means for changing the wavelength of thereading light, and a servo laser that emits servo light having adifferent wavelength from that of the reading light, an optical systemthat irradiates the reading light or servo light to the holographicrecording medium and gathers reading light generated by the holographicrecording medium by the reading light or servo light returned by theservo light by including information from the holographic recordingmedium, an imaging element that detects the reading light, a lightdetector that detects the servo light, and temperature measuring meansfor measuring the surrounding temperature of the holographic recordingmedium, wherein data information is read from the holographic recordingmedium by irradiating the reading light to the holographic recordingmedium, and data information is read by changing the wavelength of thereading light by the wavelength control means based on information onthe thermal expansion characteristic of the recording material and/orinformation on the temperature dependency of the refractive index of therecording material, which is read from the holographic recording medium,information on the reference temperature and information on thereference wavelength, and information on the surrounding temperature inreading data information, which is measured by the temperature measuringmeans, and irradiating the reading light to the holographic recordingmedium.

The information on the thermal expansion characteristic of the recordingmaterial and/or the information on the temperature dependency of therefractive index of the recording material, the information on thereference temperature and the information on the reference wavelengthmay be read by irradiating the servo light with the wavelength which isnot sensible by the recording material.

Data information may be read by changing the wavelength λr of thereading light to the range of the values calculated by one of:0.99*(1+(α−β)*(Tr−Tc))*λc)≦λr≦1.01*((1+(α−β)*(Tr−Tc))*λc)  [7];0.99*((1+α(Tr−Tc))*λc)≦λr≦1.01*((1+α(Tr−Tc))*λc)  [8]; and0.99*((1−β(Tr−Tc))*λc)≦λr≦1.01*((1−β(Tr−Tc))*λc)  [9]

where information on the thermal expansion characteristic of therecording material is a linear expansion coefficient α, information onthe temperature dependency of the refractive index of the recordingmaterial is a primary coefficient β of the ratio of a change inrefractive index in accordance with a change in temperature, informationon the reference temperature is a temperature Tc, information on thereference wavelength is a wavelength λc, and information on thesurrounding temperature in reading data information is a temperature Tr,and irradiating the reading light to the holographic recording medium.

Data information may be read by adjusting the optical zoom in accordancewith a change in wavelength of reading light by the wavelength controlmeans.

According to the embodiments, a holographic reading system that recordsinformation on a thermal expansion coefficient and a temperaturecharacteristic of a change in refractive index on a medium in advanceand uses a holographic recording medium that records in writinginformation on the wavelength and temperature in writing allows theestimation of the wavelength of reading light satisfying a diffractioncondition even when the surrounding temperatures of the optical head inwriting and reading differ. Thus, data information can be read fast andin a stable manner.

The information on the wavelength of writing light and the surroundingtemperature in writing can be read with a wider wavelength range byusing a holographic recording medium that records the information on thewavelength and temperature in data writing in larger pixels than aminimum pixel included in normal data information.

The holographic writing system using a holographic recording medium thatrecords information on a reference wavelength and reference temperaturewithin the medium in advance allows the adjustment of the wavelength ofwriting light based on the reference temperature and the surroundingtemperature in writing.

The holographic reading system that uses a holographic recording mediumthat records information on a reference wavelength and referencetemperature within the medium in advance can eliminate the necessity toconsider the surrounding temperature in writing when data information isread and allows the estimation of a proper wavelength for reading datainformation only by using the surrounding temperature in reading.Furthermore, when the information on a reference wavelength andreference temperature is recorded within a medium in advance, theholographic writing system can eliminate the necessity to record writingconditions (such as the surrounding temperature and wavelength) by aholographic recording scheme. In addition, the holographic readingsystem can eliminate the necessity to read the writing conditions by aholographic recording scheme in reading. Thus, the wavelength of readinglight can be estimated more securely.

Additional features and advantages are described herein, and will beapparent from, the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a section diagram showing a construction of a holographicrecording medium according to an embodiment.

FIG. 2 is a schematic diagram showing a construction of an optical headof a holographic writing/reading system according to an embodiment.

FIG. 3 is a schematic diagram showing a state of irradiation andreflection of light to/from a holographic recording medium according toan embodiment.

FIG. 4 is a diagram showing a pattern in a space modulating element inrecording.

FIG. 5 is a diagram showing a pattern in a space modulating element inreading.

FIG. 6 is a diagram showing a diffracted light pattern reproduced undera diffraction condition.

FIG. 7 is a diagram showing a pattern by which information on thesurrounding temperature (Tw) in recording and the wavelength (λw) ofwriting light is recorded on a data page.

FIG. 8 is a diagram showing a pattern by which information on thesurrounding temperature (Tw) in recording and the wavelength (λw) ofwriting light is recorded on a special page, which is different from adata page.

DETAILED DESCRIPTION

A construction of a holographic recording medium according to anembodiment of the invention will be described below.

FIG. 1 is a section diagram showing a construction example of aholographic recording medium according to an embodiment of theinvention.

A holographic recording medium 10 has emboss pits (or guide channel) 8for providing address information or servo on a clear substrate 1 ofglass or polycarbonate, for example, and a reflector (not shown) coatedwith AlTi thereon. The holographic recording medium 10 further has astack of a gap layer 2, a wavelength filter 3, a protective layer 4, arecording layer 5, a protective layer 6 and a light transmitting layer 7in this order. The gap layer 2 has a plastic film attached onto thereflecting layer with a UV hardening resin or adhesive by spin-coating.The wavelength filter 3 is a wavelength-selective reflector.

Here, the wavelength filter 3 is an optical multi-layer film designed toallow red laser light to pass through for obtaining address informationor servo from the emboss pits 8 and reflect green or blue laser lightfor recording a hologram. For example, the wavelength filter 3 is formedby placing a high-refractive layer such as TiO₂ and Nb₂O₅ and alow-refractive layer such as MgF₂ and SiO₂ next to each other andrepeatedly performing a vacuum thin film forming technology such asvapor-deposition and sputtering with high precision.

The protective layer 4 is for protecting the wavelength filter 3 from arecording material included in the recording layer 5 and does notimportantly react with a recording material. For example, the protectivelayer 4 may be formed by spin-coating a UV hardening resin coating andthen UV-hardening or by attaching thereto a plastic film with anadhesive.

The recording layer 5 includes a recording material such as aphotopolymer and records data information by a light interferencepattern.

The protective layer 6 prevents a plastic substrate of thelight-transmitting layer 7 and the recording layer 5 from the directcontact if the light-transmitting layer 7 includes the plasticsubstrate. The protective layer 6 may not be provided if thelight-transmitting layer 7 contains glass.

The light-transmitting layer 7 includes a glass or plastic substrate ofpolycarbonate, for example, and preferably contains a material having alow optical anisotropy.

The holographic recording medium 10 records in advance information on athermal expansion characteristic of a recording material included in therecording layer 5 and/or temperature dependency information on therefractive index of the recording material.

In this case, the information on the thermal expansion characteristic ofa recording material is preferably a linear expansion coefficient in atemperature area in an actually used environment of the holographicrecording medium 10. The temperature dependency information of therefractive index of a recording material is preferably a primarycoefficient of the ratio of the temperature change of the refractiveindex in the temperature area in an actually used environment of theholographic recording medium 10.

Preferably, information on the thermal expansion characteristic of arecording material and/or temperature dependency information on therefractive index of the recording material are recorded as the embosspits 8 on the substrate 1.

Preferably, the holographic recording medium 10 according to anembodiment of the invention records either:

[Information 1]: data information recorded in the recording layer 5 andinformation on the surrounding temperature of the holographic recordingmedium in data information writing and on the wavelength of writinglight,

where the information on the surrounding temperature of the holographicrecording medium in data information writing and the wavelength ofwriting light may be recorded in the recording layer 5 by a holographicrecording scheme, and the information on the temperature and recordingwavelength in data information writing is preferably recorded in largerpixel units than a minimum pixel unit used for data informationrecording, or

[Information 2]: information on a reference temperature and informationon a reference wavelength for adjusting the wavelength of writing lightfor writing data information to the recording layer 5 or the wavelengthof reading light for reading data information from the recording layer5,

where reference temperature information and reference wavelengthinformation are further preferably recorded as the emboss pits 8 on thesubstrate 1.

Next, constructions of a holographic writing system and holographicreading system according to an embodiment of the invention will bedescribed. FIG. 2 is an optical block diagram showing a construction ofan optical system (optical head) 100 used for a holographicwriting/reading system also functioning as a holographic writing systemand holographic reading system according to an embodiment of theinvention.

FIG. 2 shows a writing/reading laser 11.

An embodiment of the invention includes a wavelength control unit (notshown) that changes the wavelength of laser light emitted from thewriting/reading laser 11. A single mode may be implemented here byproviding an external resonator in a semiconductor laser (that is, thewriting/reading laser 11) with a wavelength of 405 nm, for example (asdisclosed in JP-A-2005-183426, JP-A-2005-175050, JP-A-2005-175049,JP-A-2005-167008 and JP-A-2005-159104). Alternatively, a diffractiongrating (for modulating the refractive index) may be built in an elementof the semiconductor laser for electrically controlling the refractiveindex and changing the wavelength (which implements a DFB laser or a DBRlaser).

FIG. 2 further shows a space modulating element 12 for creating atwo-dimensional contrast data pattern. Generally, a hologram causes theinterference of two optical fluxes of signal light and reference light,and the interference fringes are recorded on a recording medium.According to an embodiment of the invention, one space modulationelement 12 divides light from the writing/reading laser 11 into a partcorresponding to signal light and a part corresponding to referencelight and emits them as signal light and reference light (writing light)L11. The interference fringes of them are recorded in the recordinglayer 5. In reading, the part corresponding to the reference light isonly irradiated to the recording layer 5 as the reference light (readinglight) L11, which results in diffracted light.

FIG. 2 further shows a polarized light beam splitter 13 having anS1-plane which allows P-polarized light to pass through and reflectsS-polarized light. The polarizer is adjusted such that the light exitingfrom the space modulating element 12 can be the P-polarized light on theS1-plane, and the light exiting from the space modulating element 12 canpass through the S1-plane.

FIG. 2 further shows a ¼ wave plate 14 that causes the light incident onthe holographic recording medium 10 to be circularly polarized light.The ¼ wave plate changes circularly polarized light (reading light) L12reflected from a disk (that is, the holographic recording medium 10here) to S-polarized light, and the light beam of the S-polarized lighttherethrough is reflected by the polarized light beam splitter 13 and iscaptured by an imaging element 17 such as a CCD and a C-MOS.

FIG. 2 further shows a dichromic mirror 15 designed to reflect, by aplane S2, the signal light and reference light (writing light) orreference light (reading light) L11 and reading light L12 (blue light)and allows servo light L21 and servo light L22 (red light) to passthrough.

FIG. 2 further shows a condenser 16 that gathers the writing light orreading light L11 (blue light) and servo light L21 (red light) withinthe holographic recording medium 10.

FIG. 2 further shows a servo laser 18 that emits light to be the servolight L21 and emits light with a wavelength which is not sensible by therecording layer 5, unlike the wavelength of the writing/reading laser11. For example, a semiconductor laser with a wavelength of 650 nm maybe used.

FIG. 2 further shows a ½ wave plate 19 that adjusts the light emittedfrom the servo laser 18 to be S-polarized light incident by a polarizedlight beam splitter 20.

The polarized light beam splitter 20 allows P-polarized light to passthrough and reflects S-polarized light by the S-plane.

FIG. 2 further shows a ¼ wave plate 21 that causes the light incident onthe holographic recording medium 10 to be circularly polarized light(servo light L21) and changes the circularly polarized light (servolight L22) reflected from the holographic recording medium 10 toP-polarized light.

FIG. 2 further shows a servo information detector 22 that receives servolight including the information of the emboss pits 8 reflected back fromthe holographic recording medium 10. More specifically, the servo lightreflected back from the holographic recording medium 10 passes throughthe dichromic mirror 15 and then through the ¼ wave plate 21 and becomesP-polarized light. The P-polarized light passes through the polarizedlight beam splitter 20 and is received by the servo information detector22.

A holographic writing system and holographic reading system according toembodiments of the invention include a temperature measuring unit (notshown) that measures the surrounding temperature of the holographicrecording medium 10.

In order to write on the holographic recording medium 10, writing light(signal light and reference light) L11, which is laser light with a bluewavelength, from an optical head 100 is irradiated from thelight-transmitting layer 7 side as shown in FIG. 3 so as to create aninterference pattern within the recording layer 5 of the holographicrecording medium 10. The signal light and reference light of theincident writing light L11 interfere with each other in the recordinglayer 5 and thus create an interference pattern, which implementswriting. The writing light L11 then passes through the recording layer 5and enters to the wavelength filter 3 but is reflected by the wavelengthfilter 3 and becomes reflected light.

FIG. 4 shows a pattern in the space modulation element 12 in writing.The pattern may be divided into a data signal area at the center and areference light area outside. The reference light area generally has afixed pattern.

In order to write on the holographic recording medium 10, writing lighthaving a spatial distribution of strength of the pattern shown in FIG. 4is gathered into the recording layer 5 of the holographic recordingmedium 10 by the condenser 16. Then, a distribution of refractive indexreflecting the distribution of strength occurs in the recording layer 5,whereby writing is performed.

In order to read from the holographic recording medium 10, the readinglight (reference light) L11, which is laser light with a bluewavelength, is focused onto the recording layer 5 of the holographicrecording medium 10 from the optical head 100 and is irradiated from thelight-transmitting layer 7 side. The incident reading light L11 becomesdiffracted light in accordance with the interference pattern on therecording layer 5 and is emitted from the surface of the holographicrecording medium 10 as reading light L12.

More specifically, the light through the pattern in the space modulatingelement 12 as shown in FIG. 5 is caused to enter to the holographicrecording medium 10 as the reading light L11. When the reading light L11satisfies the distribution of refractive index and diffraction conditionwithin the recording layer 5, the diffracted light pattern as shown inFIG. 6 can be obtained, whereby data information is read.

Here, the satisfaction of the diffraction condition is significantlyimportant for reading data information. Especially, a difference insurrounding temperature of the holographic recording medium 10 betweenwriting and reading causes thermal expansion/contraction and/or a changein refractive index in the recording layer 5. Then, when reading isperformed with reading light with the same wavelength as that inrecording, the diffraction condition is not satisfied, which preventsthe data reading.

Laser light (red light) with a red wavelength, as the servo controllinglight (servo light) L21, from the optical head 100 is irradiated fromthe light-transmitting layer 7 side so as to focus on the emboss pits 8(more strictly, the reflector thereabove). In this case, the servo lightL21 is reflected by the reflector through the light transmitting layer7, protective layer 6, recording layer 5, protective layer 4, wavelengthfilter 3 and gap layer 2 and then, as the reflected light (servo light)L22 including the information in accordance with the emboss pits 8, isemitted from the surface of the holographic recording medium 10 throughthe gap layer 2, wavelength filter 3, protective layer 4, recordinglayer 5, protective layer 6 and light-transmitting layer 7 again (FIG.3).

EXAMPLES

As examples of the invention, the writing/reading to/from theholographic recording medium 10 employing the holographicwriting/reading system shown in FIG. 2 will be described below.

Here, the holographic recording medium 10 includes:

substrate 1: a plastic substrate of polycarbonate having the emboss pits8 on the surface and an AlTi film of a thickness of 20 nm as a reflectorthereon,

gap layer 2: formed by crimping a polycarbonate (PC) film (of a totalthickness of 100 μm) having a pressure-sensitive adhesive layer,

protective layer 4: formed by crimping PC/SiO₂ laminated film (of atotal thickness of 100 μm) having a pressure-sensitive adhesive layer,

protective layer 6: an SiO₂ layer (of a thickness of 20 nm), and

light transmitting layer 7: a PC substrate of a thickness of 0.6 mm.

In the optical head 100, a semiconductor laser with a wavelength of 405nm to which an external resonator is installed to work in a single modeis used as the writing/reading laser 11 and a semiconductor laser with awavelength of 650 nm is used as the servo laser 18.

Example 1 An Example in which the Temperature and Wavelength in Writingare Recorded as Holographic Data

As described above, a difference in surrounding temperature of theholographic recording medium 10 between writing and reading causesthermal expansion/contraction and/or a change in refractive index in therecording layer 5. Then, when reading is performed with reading lightwith the same wavelength as that in recording, the diffraction conditionis not satisfied, which prevents the data reading. However, thediffraction condition can be satisfied by changing the wavelength inreading. In this case, for example, if the thermal expansioncoefficient, material characteristic such as a temperature-dependentcharacteristic of the refractive index, temperature in writing,wavelength and temperature in reading of the recording layer 5 areavailable, the approximate wavelength in reading that satisfies thediffraction condition can be estimated.

The thermal expansion coefficient, and temperature-dependentcharacteristic of the refractive index of the recording layer 5 may beconsidered as being inherent to the recording material thereof and maybe recorded on the holographic recording medium 10 as the emboss pits 8in advance. For example, like an optical disk such as an MO and a DVDhaving a place where the information on a medium such as an optimumwriting power is recorded in advance, the holographic recording medium10 according to this example of the invention may have such a placewhere the information on the thermal expansion coefficient andtemperature-dependent characteristic of the refractive index is recordedin advance. The information of the emboss pits 8 is preferably read bythe wavelength, which is not sensible by the recording layer 5. Forexample, servo light may be used.

Since the thermal expansion coefficient itself of the recording materialis also temperature-dependent, some thermal expansion coefficient valuesin an assumed used temperature environment, such as 5° C. to 55° C., aredesirably recorded. However, the representation with one value (α) inthe temperature range does not have a large effect on the estimation ofthe wavelength of reading light that satisfies the diffractioncondition.

The temperature dependency of the refractive index may be approximatedas that the refractive index changes substantially linearly inaccordance with a change in temperature in a certain temperature range.Thus, a factor of proportionality (which is a temperature-characteristiccoefficient of the refractive index here) exists for each temperaturelike the thermal expansion coefficient, and the temperaturecharacteristic coefficients of the refractive indices for sometemperatures are desirably recorded. However, like the case of thethermal expansion coefficient, the representation with one value (β)does not have an effect on the estimation of the wavelength of readinglight.

On the other hand, information on the temperature of the holographicrecording medium 10 in writing and the wavelength of writing light maynot be recorded in the emboss pits 8 in advance unless the recording isnot specified in the system. Therefore, the information is written inthe recording layer 5 in writing as holographic data information. Thetemperature of the holographic recording medium 10 may approximate tothe surrounding temperature of the holographic recording medium 10,which is measured by a temperature measuring unit.

Generally, in order to increase the data amount within one page, onedata pixel unit of the data pattern as shown in FIG. 6, of user data isreduced as much as possible. However, the data information may thereforenot be read due to a slight difference in the diffraction condition.

The wavelength for reading the information (information 1) on thesurrounding temperature (Tw) of the holographic recording medium 10 inwriting and the wavelength (λw) of writing light, which is recorded forestimating the wavelength of reading light, does not typically satisfythe diffraction condition. Thus, the secure reading of the informationis important even with the wavelength, which is slightly different fromthe wavelength satisfying the diffraction condition. Thus, as shown inFIG. 7, the information 1 is preferably recorded in larger data pixelunits than data pixel units of a general data pattern. Alternatively, asshown in FIG. 8, a special page for recording the information 1 may beprovided in addition to a normal data page.

However, even with such larger data pixels for recording the informationon the surrounding temperature (Tw) of the holographic recording medium10 in writing and the wavelength (λw) of writing light, the wavelengthlargely deviating from the wavelength satisfying the diffractioncondition may not be read. In this case, the information 1 may berequired to read by changing the laser wavelength simultaneously.

When the thermal expansion coefficient (α) of a recording materialincluded in the recording layer 5, the temperature characteristiccoefficient (β) of the refractive index, the surrounding temperature(Tw) in writing, the surrounding temperature (Tr) in reading and thewavelength (λw) of writing light in writing are available in reading,the wavelength (λr) of reading light satisfying the diffractioncondition of the recording layer 5 can be estimated as:λr=(1+(α−β)*(Tr−Tw))*λw

where a higher-order term is ignored.

There is no problem if the wavelength of reading light in reading can beadjusted as estimated above, but the adjustment may not be implemented.However, accurate data reading may require the wavelength within atleast ±1% from the wavelength satisfying the diffraction condition, andthe range is desirably within ±0.5% under an especially strictdiffraction condition that a lens with a high numerical aperture isused, for example.

Apparently, if the temperature characteristic coefficient of therefractive index of a recording material is sufficiently lower than thethermal expansion coefficient, the influence of the thermal expansionmay be only required to consider. In this case, the temperaturecharacteristic coefficient of the refractive index does not have to berecorded on a medium in advance, and the wavelength of reading lightsatisfying the diffraction condition may be considered as:λr=(1+α(Tr−Tw))*λw

Conversely, if the temperature characteristic coefficient of therefractive index of a recording material is dominant, the influence of achange in refractive index may be only required to consider. In thiscase, the thermal expansion coefficient does not have to be recorded ona medium in advance, and the wavelength of reading light satisfying thediffraction condition may be considered as:λr=(1−β(Tr−Tw))*λw

A partial area of each data page may be used, as shown in FIG. 7, forthe information on the surrounding temperature (Tw) in writing and thewavelength (λw) of writing light, which may be required here.Alternatively, as shown in FIG. 8, a special page for recording theinformation may be provided in addition to a normal data page.

An optical zoom is preferably adjusted when data information is read bychanging the wavelength of reading light. More specifically, an opticalzoom of λw/λr times allows reading data information in equal size tothat in writing.

In a specific example of this example, the wavelength λr of readinglight in reading is:λr=411.5 nm

where writing with the surrounding temperature Tw=25° C. in writing andthe wavelength λw=405 nm of writing light is performed on a recordingmaterial with the linear expansion coefficient α=0.001/° C. and thetemperature characteristic coefficient β=−0.00007/° C. of the refractiveindex, and reading is performed at a surrounding temperature of 40° C.

Notably, in this specific example, the wavelength of reading light λr inreading isλr=411.1 nm

where the linear expansion coefficient is sufficiently large incomparison with the temperature characteristic coefficient of therefractive index and β=0,

which is not significantly different from the case where β isconsidered. Therefore, the value of the linear expansion coefficient maybe only recorded on the holographic recording medium 10 by ignoring achange in refractive index.

Example 2 An Example in which the Reference Temperature and ReferenceWavelength are Recorded in Advance

The information on the reference wavelength and reference temperature isrecorded in the emboss pits 8 of the holographic recording medium 10 inadvance in addition to the information on the thermal expansioncoefficient and the temperature dependency of the refractive index of arecording material as described in Example 1. In other words, theapproximate wavelength in writing and reading that satisfies thediffraction condition can be estimated when a material characteristicsuch as the thermal expansion coefficient and the temperature dependencycharacteristic of the refractive index of the recording layer 5, thereference wavelength and reference temperature and the surroundingtemperature in writing or reading are available.

First, data information may be recorded by reading the thermal expansioncoefficient (α) of a recording material included in the recording layer5, the temperature characteristic coefficient (β) of the refractiveindex, the reference wavelength (λc) and reference temperature (Tc) fromthe holographic recording medium 10, reading the surrounding temperature(Tw) from a holographic writing/reading system, and adjusting thewavelength (λw) of writing light in writing to the value calculated by:λw=(1+(α−β)*(Tw−Tc))*λc

In this case, the temperature characteristic coefficient of therefractive index is sufficiently small in comparison with the thermalexpansion coefficient, the influence of the thermal expansion may beonly required to consider. The temperature characteristic coefficient ofthe refractive index does not have to be recorded on the holographicrecording medium 10 in advance. Therefore, the wavelength of writinglight in writing may be considered as:λw=(1+α(Tw−Tc))*λc

Conversely, if the temperature characteristic coefficient of therefractive index is dominant, the influence of a change in refractiveindex may be only required to consider. In this case, the thermalexpansion coefficient does not have to be recorded on the holographicrecording medium 10 in advance. Therefore, the wavelength of writinglight in writing may be considered as:λw=(1−β(Tw−Tc))*λc

Writing with writing light having the wavelength allows the wavelengthof the reading light satisfying the diffraction condition for datareading at the reference temperature (Tc) to be the reference wavelength(λc) regardless of the temperature at which the data information iswritten.

On the other hand, data information may be read by reading thesurrounding temperature (Tr) from a holographic writing/reading systemand adjusting the wavelength (λr) of reading light in reading to thevalue calculated by:λr=(1+(α−β))*(Tr−Tc)*λc

In this case, the temperature characteristic coefficient of therefractive index is sufficiently small in comparison with the thermalexpansion coefficient, the influence of the thermal expansion may beonly required to consider. The temperature characteristic coefficient ofthe refractive index does not have to be recorded on the holographicrecording medium 10 in advance. Therefore, the wavelength of readinglight satisfying the diffraction condition may be considered as:λr=(1+α(Tr−Tc))*λc

Conversely, if the temperature characteristic coefficient of therefractive index is dominant, the influence of a change in refractiveindex may be only required to consider. In this case, the thermalexpansion coefficient does not have to be recorded on the holographicrecording medium 10 in advance. Therefore, the wavelength of readinglight satisfying the diffraction condition may be considered as:λr=(1−β(Tr−Tc))*λc

Also in this example, there is no problem if the wavelengths of writingand reading light can be adjusted as estimated above, but the adjustmentmay not be implemented. However, accurate reading of data informationmay require writing and reading with the wavelength within at least ±1%from the wavelength, and the range is desirably within ±0.5% under anespecially strict diffraction condition that a lens with a highnumerical aperture is used, for example.

In a specific example of this example, the wavelength λw of writinglight in writing is:λw=402.8 nm

where writing with the surrounding temperature Tw=20° C. in writing isperformed on a recording material with the linear expansion coefficientα=0.001/° C., the temperature characteristic coefficient β=−0.00007/° C.of the refractive index in a system with the reference wavelength λc=405nm and the reference temperature Tc=25° C.

On the other hand, data information recorded under these conditions isread at a surrounding temperature Tr=30° C. in reading, and thewavelength λr of reading light in reading is:λr=407.2 nm

Notably, also in this specific example, the wavelengths of writing andreading light isλw=403.0 nm and λr=407.0 nm

where the linear expansion coefficient is sufficiently large incomparison with the temperature characteristic coefficient of therefractive index and β=0,

which is not significantly different from the case where β isconsidered. Therefore, the value of the linear expansion coefficient maybe only recorded on the recording medium by ignoring a change inrefractive index.

According to this example, the wavelength of reading light may bedetermined in reading only by considering the surrounding temperature inreading independently of the surrounding temperature in writing thoughthe adjustment of the wavelength of writing light may be inconvenientlyrequired in writing. In other words, the wavelength and surroundingtemperature of writing light in writing in Example 1 do not have to berecorded by a holographic scheme every time.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A holographic recording medium comprising: a recording layer on asubstrate, which records data information in a light interferencepattern, wherein at least one of information on a thermal expansioncharacteristic of a recording material contained in the recording layerand information on temperature dependency of the refractive index of therecording material are recorded within the holographic recording mediumin advance.
 2. The holographic recording medium according to claim 1,wherein the information on a thermal expansion characteristic of therecording material is a linear expansion coefficient in an actually-usedenvironment temperature area of the holographic recording medium.
 3. Theholographic recording medium according to claim 1, wherein at least oneof the information on the thermal expansion characteristic of therecording material and the information on the temperature dependency ofthe refractive index of the recording material are recorded as embosspits of the substrate.